Suggestion from a guy who has run for over 33 yrs and closing in on 70,000 miles. Slow down. Starting at 51if you don't take it easy you won't last too long. Believe it or not if you slow downbuild a strong aerobic base--let's say 1 yr. Gradually build miles. Get off the track get out on the roads. You will have a greater chance of becoming a lifetime runner. If not, the game will be over and you will then join the world's largest running club"I Use to run, but.........club." Nick